The invention relates to a method for the production of stampings with an almost smooth cutting and enlarged functional surface, especially fine blanking and/or forming a workpiece out of a flat strip, wherein the flat strip at closing is clamped between an upper part consisting of a shearing punch, a pressure pad for the shearing punch, a V-shaped projection arranged on the pressure pad, and an ejector and a lower part consisting of a cutting die, an ejector and an inner form stamp, and in the cutting zone a cut is enforced by shearing at high compressive stress, wherein the V-shaped projection has been pressed into the flat strip beforehand and a compressive stress acts on the flat strip to be cut.
The invention further relates to a device for the production of stampings with an almost smooth and enlarged functional surface, especially fine blanking and/or forming a workpiece out of a flat strip, with a tool having two parts comprising at least a main shearing punch, a pressure pad for the shearing punch, a V-shaped projection arranged on the pressure pad, an ejector, and a cutting die, wherein the flat strip is clamped between pressure pad and cutting die and the V-shaped projection is pressed into the flat strip.
It is known that fine blanking at projecting contours of parts, for example toothings or corners, often leads to tears at the cutting surfaces. This phenomenon can be observed in even greater intensity the sharper an outer contour is defined, the thicker the material to be cut and the smaller is the ductility of the material. In most of the cases in fine blanking, the cutting surface acts as functional surface, wherefore tears can be the point of origin of a breaking failure of the part under load and therefore have to be avoided.
Smooth cutting surfaces at fine blanking are achieved, when in the cutting zone, by superposition of high hydrostatic pressure is enforced, a cut by shearing, i.e. a plastic deformation. The cutting surface occurs in the shearing zone, and thus, with regard to its quality, is influenced by the material properties (K. KONDO, Industrie-Anzeiger, annual volume 39, nr. 33, p. 547 to 550). At fine blanking, the V-shaped projection, before the cutting starts, is pressed into the material of the flat strip to be cut, and thus prevents continuous flow of the material during the cutting process.
Furthermore, typical features of fine blanking parts are the edge rollover and the cutting burr. Especially in corner areas, the rollover occurs and grows with decreasing corner radius and increasing sheet thickness. The depth of the rollover can be about 30% and the width of the rollover about 40% of the sheet thickness or more (see DIN 3345, Feinschneiden, Aug. 1980). Thus, the rollover depends on material thickness and quality, so that the possibility to control it is limited and often brings about a limited functionality of parts, for example due to a lack of sharp edges of the corners at toothed parts or the caused change in the functional length of the parts.
The stamping rollover thus reduces the functionality of parts and urges the manufacturer to use a thicker raw material.
A number of solutions are known for trying to produce fine and smooth shearing surfaces by cutting under pressure (DE 2 127 495 A1), re-cutting (CH 665 367 A5), shaving (DE 197 38 636 A1) or shifting of material during the cutting (EP 1 815 922 A1).
The known solutions according to CH 665 367 A5 and DE 197 38 636 A1 do not reduce the edge rollover, but largely rework the parts, so that on the one hand, significant costs for additional machining operations and tools are required and, on the other hand, a respective loss of material due to the necessity of using thicker materials occurs.
The known shearing press according to DE 2 127 495 A1 is operated at a higher hydrostatic pressure acting on the whole area of the workpiece subjected to plastic deformation. This high pressure, especially near the edges of the tool, is created by an upper jaw having a projection. This projection, so to speak, carries out the function of the V-shaped projection not existing according to DE 2 127 495 A1. But with this known method, in the first instance, the projecting stamping burr is avoided. Also with this known solution the rollover lastly is not avoided and material volume is shifted along the cutting line, which is accompanied by an increased risk of developing tears.
In the known solution according to EP 1 815 922 A1, the workpiece is machined in a single-step setup in at least two chronologically successive steps in different cutting directions, wherein during a first cutting process in vertical working direction is cut out a semi-finished product corresponding to the geometry of the workpiece with small rollover and finally cut during at least one further cutting process in the opposite working direction. The rollover of the first partial step with this shall be filled up again at least in the corner area. But with this known method, in the first instance, the projecting stamping burr is avoided. Also with this known solution, the rollover lastly is not avoided and material volume is shifted along the cutting line, which is accompanied by an increased risk of developing tears.
At this state of the art the invention has the task to avoid the inclination to tearing at cutting surfaces and the edge rollover at fine blanking parts by systematically controlling the state of stress in the cutting zone and at the same time to economically and effectively realize the fine blanking of thicker parts with high process security.